1. Field of the Invention
This invention relates to an engine provided with precombustion chambers on the side of pistons.
2. Description of the Prior Art
A swirl chamber type engine provided with swirl chambers for the purpose of improving the combustion efficiency of the engine has been developed. The swirl chamber type engine has swirl chambers formed in a cylinder head or piston heads, communication ports via which the swirl chambers and main combustion chambers formed in cylinders communicate with each other, and fuel injection nozzles for injecting a fuel into the swirl chambers. In this engine, a gaseous mixture of an eddy current and a fuel flowing into the swirl chambers via the communication ports is formed and subjected to primary combustion, and gases, such as flames and an unburnt gaseous mixture are then ejected from the swirl chambers into main combustion chambers via the communication ports and subjected to secondary combustion. There is a combustion chamber structure disclosed in Japanese Patent Laid-Open No. 112613/1990 which constitutes an example of such a swirl chamber type engine.
Japanese Utility Model Laid-Open No. 93141/1987 discloses a piston consisting of a ceramic crown and an aluminum skirt. In this piston, a head portion of a cylindrical aluminum skirt is fitted around the outer circumferential surface of the ceramic crown in which a cavity is formed, and the outer circumferential surface of a combustion chamber and the head portion of the skirt are fixed by a cast iron ring.
Japanese Patent Laid-Open No. 26111/1993 discloses a heat insulating piston. In this heat insulating piston, the heat insulation between a piston head and a piston skirt is secured, and the cracking and scattering of a heat insulating member provided between joint portions of the two parts are prevented.
In general, the combustion temperature in a precombustion chamber is high, and, therefore, carrying out the combustion of a gaseous mixture at a high fuel-air ratio is effective as measures for reducing the production of NOx. In order to burn a gaseous mixture at a high fuel-air ratio in an advantageously constructed engine therefor when a combustion temperature is high, using a precombustion chamber type engine is effective. In order to increase a combustion speed in a precombustion chamber type engine formed by providing a precombustion chamber in a conventional engine, it is necessary that a cross-sectional area of a communication port via which the precombustion chamber and a main combustion chamber communicate with each other be reduced so as to increase the velocity of flow of an eddy current and that of a jet current entering the main combustion chamber. However, when the cross-sectional area of the communication port is reduced, the flow rate of a gaseous mixture ejected from the precombustion chamber into the main combustion chamber increases but a throttle loss becomes large. Moreover, the combustion of a gaseous mixture in the primary combustion chamber is not sufficiently carried out.
When the cross-sectional area of a communication port in an engine, which is provided at a circumferential portion of each cylinder therein with a communication port which communicates a main combustion chamber and a precombustion chamber with each other, is increased so as to reduce a throttle loss, the penetration of injection of gases, such as flames and an unburnt gaseous mixture ejected from the precombustion chamber into the main combustion chamber becomes small, so that the gases do not reach the central portion of the cylinder. In order to improve the thermal efficiency of the engine, it is important to complete the combustion of a gaseous mixture in a short period of time by improving the air utilization rate in the main combustion chamber, and prevent the occurrence of white smoke.
In a swirl chamber type engine, the cross-sectional area of a communication port communicating a precombustion chamber and a main combustion chamber with each other is set small so as to solve these inconveniences. This causes a throttle loss due to the communication port to occur, and an engine output to decrease. In general, a communication port communicating a main combustion chamber and a precombustion chamber with each other is called a reentrant type communication port and provided solely in the central portion of a cylinder in a direct injection type engine, and solely in an outer circumferential portion of a cylinder in a precombustion chamber type engine. Accordingly, a distance over which an injection current has to run becomes long, and the mixing of a fuel with air in a main combustion chamber is not carried out sufficiently to cause HC and smoke to occur.